Anderson Janotti
Materials Department
University of California
Santa Barbara, CA 93106
Phone:(805)893-7499 Fax:(805)893-5029
Semiconductors comprise a broad class of materials whose electrical conductivity can be tuned by the addition of a relatively small amount of impurities. The conductivity can be dominated by electrons (n-type) or holes (p-type). The combination of n-type with p-type in semiconductor junctions allow for the design of electronic switches (transistors), light-emitting diodes, lasers, and photovoltaic solar cells. My research interests are oxide and nitride semiconductors for solid state lighting and solar cells. I am interested in applying first-principles methods based on Density Functional Theory to study materials properties, their defects, and hydrogen-related phenomena in solids.
Hydrogen multicenter bonds
Beyond the bonds that bind
Under the right circumstances, hydrogen can form multicenter bonds, where one hydrogen atom simultaneously bonds to as many as four or six other atoms. Tested for hydrogen in metal oxides, the discovery could have a broad range of technological impact. Metal oxides are widely used in everything from sunscreen to sensors. Hydrogen can replace an oxygen atom and form a multicenter bond with adjacent metal atoms. For example, in ZnO, hydrogen equally bonds to the four surrounding Zn atoms, becoming fourfold coordinated. These multicenter bonds are highly stable and explain previously puzzling variations in conductivity as a function of temperature and oxygen pressure. The results suggest that hydrogen can be used as a substitutional dopant in oxides, a concept that is counterintuitive and should be of wide interest to researchers. The research is available today in the advance online publication of Nature Materials. read the article in Nature Materials, EurekAlert, UCSB School of Engineering, Chemistry World.
Zinc oxide for optoelectronics
The role of point defects
Controlling the conductivity level and conductivity type in ZnO and related alloys is a fundamental step towards their utilization in optoelectronic devices. As-grown ZnO is n-type, with high levels of conductivity. The cause of this unintentional conductivity is widely debated. Because of its dependence on oxygen partial pressure and the fact that metal oxides are usually oxygen deficient, the unintentional n-type conductivity in ZnO has long been attributed to oxygen vacancies. In contrary to the conventional wisdom, we have shown that oxygen vacancies are not responsible for n-type conductivity. Oxygen vacancy is a deep donor, with ionization energy of 1 eV, and very high formation energy in n-type conditions. Our results for the calculated configuration coordinate diagrams elegantly explain recent optically detected paramagnetic- resonance measurements of oxygen vacancies in ZnO [Appl. Phys. Lett. 87, 122102 (2005)].
Hydrogen dilute nitride alloys
Nitrogen stabilizes H2* complexes in dilute GaAsN and GaPN alloys, where one H atom bonds to N and the other H atom bonds to a nearest neighbor Ga. Although the H2* complex is not stable in GaAs, GaP or GaN, the electronegativity difference between As or P and N and the strong N-H chemical bond leads to the stability of H2* in dilute GaAsN and GaPN alloys. The H2* complex is the key to the restoration of the GaAs band gap in GaAsN alloys. [Phys. Rev. Lett. 89, 086403 (2002); Phys. Rev. Lett. 88, 125506 (2002)]
Nickel-based superalloys
Larger atoms can move faster in metals
Understanding and controlling the diffusion rates of transition-metal solutes in nickel is fundamental in designing new and novel grades of nickel-based super alloys for ultra-high temperature structural applications, such as, jet and rocket propulsion (turbofans for the Boeing 777 and the Space Shuttle main engines). We established the counterintuitive idea that larger atoms can diffuse much faster than smaller atoms in transition metals. Interdiffusion rates can differ by up to four orders of magnitude when comparing elements at the extremities with elements in the middle of the 4d and 5d series of the Periodic Table. [Phys. Rev. Lett. 92, 85901 (2004)].
Thermoinduced magnetization in antiferromagnetic nanoparticles
Thermoinduced magnetization (TiM) is a ferromagnetic response predicted to occur in nanoparticles of normally antiferromagnetic material. Monte Carlo simulations for the magnetization and susceptibility indicate that TiM is an intrinsic property of the antiferromagnetic Heisenberg model below the Néel temperature. The dependence of TiM on the volume is found to be a function of both temperature and anisotropy, in contrast to the original theory for which the volume dependence is independent of temperature and anisotropy. [Phys. Rev. B 72, 140405(R) (2005)].